(1) Field of the Invention
The present invention relates to an image processing method, an image processing apparatus and a storage medium storing a program, and more particularly, to a color signal generation processing technique suitably used for an image processing function of an image forming apparatus such as a copying machine, a printer, a FAX or a compound machine having these functions in a compound manner.
(2) Description of the Related Art
In an image forming apparatus, such as a color copying machine, for forming an image on a specified recording medium, there is adopted a mechanism which creates, based on an input image signal expressing a processing object image having gradation, an output color signal corresponding to a color material used in an output device.
For example, when a color image is color-printed by an electrophotographic system or the like, in general, four-color printing with yellow (Y), magenta (M), cyan (C) and black (K) is often used. On the other hand, a general color signal is a color signal in a three-dimensional color space, such as a Lab, Luv color space independent of a device, or an RGB color space used for a monitor signal or the like. Accordingly, in the case where a color image is color-printed, it is necessary to convert the color signal in the three-dimensional color space into a four-dimensional color space.
For example, a mechanism is known in which, when an output color signal is generated, a drawing direction of a line segment is judged, a screen with an angle different from the drawing direction of the line segment is applied to a color whose disappearance should be avoided, and the screens are switched according to the drawing direction of a character or a line, so that the disappearance of a thin line is prevented.
Besides, another mechanism is also known that the amount of black is determined so that coverage limitation is satisfied and color gamut becomes maximum. By doing so, the color gamut which can be reproduced by four colors and satisfies the coverage limitation is effectively used, unnecessary color gamut compression is prevented, and conversion to a four-color color signal containing black to reproduce an objective color signal can be performed at high precision. In addition, color reproducibility can be improved by using the converted four-color color signal, or by using a multidimensional conversion table obtained by associating the converted four-color color signal with the objective color signal or conversion coefficients.
However, in the mechanism described above, although consideration is given to the disappearance of the thin line, in other words, the reproducibility of the thin line, two kinds of image attribute signals, that is, the character/line and the drawing direction are needed, and the data amount is increased. Besides, there is a disadvantage that there occurs a stepped part due to a difference in screen type.
Besides, in another mechanism described above, although the amount of black is determined so that the coverage limitation is satisfied and the color gamut becomes maximum, consideration is not given to the thin line reproducibility. Thus, although the color gamut is widened, the thin line can disappear. That is, the color signal to satisfy both the thin line reproducibility and the color reproducibility can not be generated.